<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    *{
      margin: 0;
      padding: 0;
    }
    .left{
      float: left;
      width: 100px;
      height: 100px;
      background-color: brown;
    }
    .right{
      width: auto;
      margin-left: 100px;
      height: 100px;
      background-color: chartreuse;
    }
  </style>
</head>
<body>
  <div class="demo">
    <div class="left"></div>
    <div class="right"></div>
  </div>
</body>
</html>